The present invention relates generally to an organic light emitting device (OLED), and more particularly to multiple capping layers on an electrode to provide a passivation region for the OLED to prevent formation of dark spots.
Organic light emitting devices (OLEDs) represent a promising technology for display applications. A typical organic light emitting device includes a transparent or a substantially transparent substrate; a transparent or substantially transparent first electrode, which usually acts as a hole-injecting anode; a luminescent region comprising an organic electroluminescent material; and a second electrode, which usually acts as an electron injecting cathode.
When a voltage is applied across the first and second electrodes, electrons are injected from the cathode into the luminescent region and holes are injected from the anode into the luminescent region. The holes and electrons recombine in the luminescent region, which emits light through the anode and through the substrate.
To achieve efficient electroluminescence, some known organic light emitting devices include in the luminescent region, multiple layers, such as, a layer comprising a hole transport material (HTM) adjacent to the anode, a layer comprising an emitter material (EM) which can also have charge transport properties, and/or a layer comprising an electron transport material (ETM) Examples of these organic light emitting devices can, for instance, be found in U.S. Pat. Nos. 4,356,429, 5,539,507, 4,720,432 and 4,769,292, the embodiments of which are included herein by reference in their entirety. During operation, an applied electric field causes positive charges (holes) and negative charges (electrons) to be respectively injected from the anode and the cathode in the luminescent region to recombine in the EM layer and thereby produce light emission.
Electroluminescence can be obtained from organic light emitting devices containing mixed layers in the luminescent region, for example, layers in which both a hole transport material and an electron transport material are mixed together in one single layer. Examples of these organic light emitting devices can be found, for instance, in U.S. Pat. Nos. 5,853,905, 5,925,980, 6,114,055, 6,130,001, 6,392,250, and 6,392,339, the embodiments of which are included herein by reference in their entirety.
Dark spot propagation is the propagation of non-emissive, i.e. dark, areas in an organic light emitting device. Dark spot propagation destroys the emission capability of light emitting devices such as light emitting devices used in image displays, light-emitting bars used for printing, and light emitting devices used in various illumination applications.
Although dark spot propagation can affect all types of light emitting devices, polymeric and small-molecule based organic light emitting diodes (OLEDs) suffer an increased susceptibility to dark spot propagation. Metals with low work functions, and hence high reactivity, are usually used in the cathodes of OLEDs. Due to their high reactivity, such cathode materials are unstable in ambient conditions and react with atmospheric O2 and water to form non-emissive dark spots in the OLED. The ambient moisture causes either adhesion failure between the cathode and the luminescent region, or oxidation of the cathode material which results in a lack of current flow at these areas, thus forming the dark spots.
A passivation layer can be formed on the cathode of an OLED to provide a moisture barrier. U.S. Pat. No. 5,059,861 has a single passivation layer of an alkaline earth metal on the cathode. U.S. Pat. No. 5,990,615 describes an organic light emitting device including an insulative (i.e. dielectric) protective layer on the cathode. U.S. Pat. No. 5,811,177 describes a protective region on the cathode consisting of a first layer composed of indium metal, a second layer composed of an organic material a third layer composed of a thermal coefficient matching material which may comprise a low work function metal which can optionally be a gettering material, a fourth layer composed of a low permeability inorganic material such as a metal and a fifth layer composed of epoxy or aluminum foil. World Patent Application Number WO 00/36661 describes an OLED having a passivation region consisting of one or two dielectric material layer(s) on the cathode, and interposed between the cathode and the one or more dielectric layers is an optional layer composed of a gettering material.
It is an object of the present invention to provide a multiple capping layer for a passivation region for an OLED cathode or anode to prevent formation of dark spots.
According to the present invention, an organic light emitting device (OLED) will have multiple capping layers on an electrode to provide a passivation region to prevent formation of dark spots. The passivation capping layers are, in sequence from the electrode, a first dielectric material layer, a metal or metal alloy layer with a work function of less than 4 eV, and a second dielectric protective layer.